1. Field of the Invention
The present invention relates to an electronic endoscope apparatus. More specifically, the present invention relates to control for improving quality of an image obtained by an electronic endoscope apparatus.
2. Description of the Related Art
When an observation target is viewed with use of an electronic endoscope apparatus, a part of the observation target near an illumination (a light emission part) of a scope of the endoscope apparatus reflects more light. Consequently, halation tends to occur in the part in an image obtained by the endoscope apparatus, because an amount of light received by an imaging device such as a CCD at a tip of the scope exceeds a dynamic range of the imaging device. Presence of halation causes the image displayed on a monitor to become hard to observe. In addition, halation tends to occur in a region of interest due to an inevitable manner of observation resulting from a shape of the observation target and a structure of the scope, which affects observation and diagnosis.
For example, the inner wall near the stomach entrance is a region of interest where inflammations, such as an ulcer, tend to be observed. When the inner wall is observed, the endoscope apparatus is generally inserted from the entrance and a tip of the scope is bent to turn toward the entrance for observation. At this time, halation tends to occur on the upper or lower side of the inner wall close to the illumination of the scope. Therefore, to observe of the entire stomach entrance, the scope is moved to be twisted with the tip thereof being bent in order to prevent halation from occurring at a part to be observed, since the direction of the tip cannot be freely changed due to the shape of the stomach and a limit resulting from the structure of the scope. Alternatively, a distance between the part to be observed and the scope is changed frequently to adjust illuminance. However, such frequent movement of the scope increases a burden on a patient to be examined, which is not preferable.
For this reason, methods have been proposed in U.S. Pat. No. 6,545,703 and Japanese Unexamined Patent Publications Nos. 2003-250761 and 6(1994)-078312, for example, to reduce halation that causes adverse effects on diagnosis. In these methods, presence of halation is automatically detected based on an average luminance value, a density histogram, or the like of an image obtained by a CCD, and the amount of light is reduced by controlling an iris installed on a light emission side of an illuminating means in the case of presence of halation.
However, according to the methods wherein the amount of light is simply reduced at the time of occurrence of halation, a part where the light cannot easily reach becomes darker and becomes hard to observe, although halation can be suppressed. As a result, the distance between an observation target and a scope, or the illuminance of the light, needs to be adjusted manually from time to time. Consequently, the frequent movement of the scope becomes a burden on a patient, and an operator is troubled with the adjustment of the illuminance.
Therefore, an image obtained by an electronic endoscope apparatus becomes preferable when the image covers a wide range that can be observed at once. In other words, an image with suppressed halation and fewer dark areas is preferable. If such an image is obtained, the burden on a patient and the trouble of illuminance adjustment can be reduced.